"setoption name debug log file value" does not close previous debug log file

[jomega-github]

Windows 10

Running stockfish in a command window and issuing the command
setoption name debug log file value c:\users\jomeg\sf.log
correctly starts a debug log to that file. However, I'd expect the command
setoption name debug log file value
to close the file. I expect that because it seems the intension of the uci doc and the code at
misc.cpp#126 which is the code to close the file.

However, the file never closes, and you cannot open another file with the setoption command either.
I know it does not close because any attempt to delete it fails (until stockfish is exited).

I believe the problem is in ucioption.cpp#167, which is...

if ( (type != "button" && v.empty())
|| (type == "check" && v != "true" && v != "false")
|| (type == "spin" && (stof(v) < min || stof(v) > max)))
return *this;

Since the type of this option is "string" and the value of v is indeed empty (verified by single stepping in a debugger)
the return happens immediately and hence the on_change() function does not get called, and hence the code
misc.cpp#126 to trigger closing the file never happens.